Physiological role and molecular mechanism of a novel growth hormone sGH system in the chicken

鸡生长激素（GH）基因通过两个转录起始位点产生两种不同的GH mRNA：垂体GH (p-GH) mRNA编码循环GH，垂体外GH mRNA编码GH细胞质异构体（s-GH）。s-GH mRNA是从GH基因的第三个内含子转录而来，该内含子由5个外显子和4个内含子组成(1)。在这里，我们首次证明了鸡生长激素基因在多种组织中双向转录，产生生长激素感（p-GH和/或s-GH） mrna和多种选择性剪接的反义转录物。采用RT-PCR技术从鸡胚眼中分离到7个具有不同外显子组成的GH-as转录本。5‘和3’ RACE表明，它们的转录始于GH感觉mRNA的poly (A)位点下游3‘处，并且每个转录本都可以通过位于p-GH mRNA启动子上游5’处的三个poly (A)信号中的任何一个进行聚腺苷化。这些GH-as转录本由至少5个外显子组成，其中大多数与GH感觉mrna的外显子重叠。有趣的是，在所有GH-as转录本中都没有观察到超过复数外显子的开放阅读框。RT-PCR分析显示，GH感觉mrna和GH-as转录本在成鸡和3日龄鸡中广泛共表达，GH感觉mrna和GH-as转录本的相对丰度随组织类型的不同而不同。此外，b细胞系DT40同时表达GH感mrna和GH-转录本，表明双向转录发生在单个细胞中。需要进行蛋白质分析；然而，这些发现表明，鸡的p-GH和/或s-GH的表达可能在转录后水平上通过GH义转录物和反义转录物的相互作用受到调节。

The chicken growth hormone (GH) gene produces two distinct GH mRNAs by alternative usage of two transcription initiation sites : the pituitary GH (p-GH) mRNA encoding circulating GH, and the extra-pituitary GH mRNA encoding a GH cytosolic isoform (s-GH). The s-GH mRNA is transcribed from the third intron of the GH gene which consists of 5 exons and 4 introns (1). Here we demonstrate for the first time that the chicken GH gene is bi-directionally transcribed in a wide variety of tissues to produce GH sense (p-GH and/or s-GH) mRNAs and multiple alternatively spliced anti-sense transcripts. Seven GH anti-sense (GH-as) transcripts with distinct exon composition were isolated by RT-PCR with RNA from embryonic chicken eyes. The 5' and 3' RACE revealed that their transcription initiates in the 3' downstream from the poly (A) site of the GH sense mRNAs, and each transcript was polyadenylated by using either of three poly (A) signals located 5' upstream from the promoter for p-GH mRNA. Those GH-as transcripts were composed of at least 5 exons, most of which overlap exons of the GH sense mRNAs. Interestingly, no open reading frame expanding over plural exons was observed in all GH-as transcripts. The RT-PCR analysis revealed that the GH sense mRNAs and the GH-as transcripts are co-expressed widely in adults and 3-day-old chickens, and the relative abundance of GH sense mRNAs and GH-as transcripts varied with tissue types. Moreover, the B-cell cell line DT40 expressed both GH sense mRNAs and GH-as transcripts, indicating that the bi-directional transcription occurs in a single cell. Protein analyses are required ; however, these findings imply the possibility that p-GH and/or s-GH expression is regulated at post-transcriptional levels by interaction between GH sense and anti-sense transcripts in the chicken.